In the continuous manufacture of electric cables having an insulation of cross-linked polymer, the polymer is extruded in cross-linkable form around the cable conductor and is then cross-linked by means of heating under pressure in a vulcanization tube. Organic peroxides are usually used to effect the cross-linking.
Heating may be effected by known methods using water vapor, a gas, a molten inorganic salt, or from hot walls with which the polymer is brought into direct contact. In all these methods the heat is transmitted from the surface of the cable to the inside, which involves high surface temperature and a temperature drop towards the center of the cable. Since polymer material has a relatively low coefficient of thermal conductivity, the production rate is limited, especially for cables with considerable insulation thickness, by the fact that the heat transmission cannot be further increased by conduction in the polymer material once the surface temperature of the cable has reached a magnitude corresponding to the decomposition temperature of the material. The resistance to thermal conductivity for a 30 m polythene insulation with a surface temperature of 250.degree. C., for instance, is as much as about 250 cm.degree.C.sec/cal and for a 5 mm insulation layer about 50 cm.degree.C.sec/cal. The polymer which is to form the cable insulation must be heated sufficiently for the inner regions of the insulation to achieve the requisite degree of cross-linking.
Electric cables intended for high operatiing voltages are normally provided with an electrically-conducting layer (nner semi-conductor) extruded around the conductor and a conducting layer (outer semi-conductor) extruded around the outer surface of the insulation, both of these layers being of polymer material, the conducting properties of which are effected by mixing in carbon black of conducting type. The carbon black makes the layers black. These conducting layers are usually cross-linked at the same time as the polymer in the insulation is cross-linked.
It is also known to provide the heating for the cross-linking by means of radiation energy. With this known method the cable conductor with extruded cross-linkable polymer, but without outer conducting layer, is passed through a vulcanization tube of steel, the walls of which are heated to 400.degree.-500.degree. C. and which contains an inert gas, to effect the requisite pressure on the extruded polymer. Heat is then transmitted to the cable through radiation and convection. With transparent polymers in the cable insulation this method gives a certain increase in the production rate, explained by the fact that the thermal radiation can penetrate the polymer so that radiation energy is transmitted to the inner part of the cable. Most of the radiation energy, however, is taken up by the surface layer of the cable.
According to the present invention it has proved possible to attain a considerably higher production rate than was possible with methods known hitherto. A particularly noticeable increase in the production rate has been achieved for cables with thick insulation. This is because according to the invention it is possible to effectively transmit heat to the inner parts of the cable and thus achieve rapid heating of the entire cross-section of the cable as well as rapid cross-linking through the entire cross-section. Furthermore, as will be described in more detail later on, the present invention also permits continuous inspection of the cable conductor or inner conducting layer and insulation during manufacture so that corrections can be made in the manufacturing process as necessary.